DE2214398A1 - Method and arrangement for quickly obtaining the initial convergence of the gain settings in a transversal equalizer - Google Patents

Description

Western Electric Company Inc. ^ - Cl H o33

New York, N.Y., USA Mueller 1 - 3

Method and arrangement for quickly obtaining the initial convergence of the gain settings in a transversal equalizer.

The invention relates to a method and arrangements for implementation the method for quickly obtaining the initial convergence of the gain settings in a transversal equalizer a synchronous data transmission system with a tapped Delay line that has an adjustable attenuator for each tap and with a summing circuit for selective Combination of the attenuated signals at the taps to form an equalized output signal.

The inter-symbol influence as a result of the overlap of the (response) frequency components of successive signals is a serious drag on synchronous digital data transmission over voiceband telephone channels. It is therefore automatic equalization is required if maximum speed

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ability data over such a channel with unknown channel parameters be transmitted. The distorter generally consists of a transversal filter with adjustable tap coefficients or gain settings. These coefficients are set to initial values derived from test pulses or trains that are transmitted via the transmission channel used and later by a setting algorithm that relates to the observations of the received message signals is adaptive. Since the distortion characteristics of telephone channels over a very wide range is variable, depending on factors such as circle length and composition of the media, is an adjustment period Required before the message data transmission to the initial values for the adjustment of the distortion to the unknown channel distortion to determine. Usually a group of pulses or a pseudo-random test pattern is transmitted in a set-up mode, in order to find out the channel parameters and to adjust the equalizer coefficients as precisely as possible with respect to their optimal values. The initial values, once obtained, can alternatively be retained during the subsequent message transmission or the message data itself can be monitored in order to continuously readjust the equalizer so that slow time-variant channel parameters tracked in an adaptive mode of operation will.

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In U.S. Patent 3 292 110 automatic equalizers are described, that use an adjustment mode for initial adjustment. Finally, see U.S. Patent 3,368,168 for details adaptable equalizer described.

When the behavior of a transmission channel on a single pulse and its disturbance variables are known in the receiver, the optimal Tap gains can be theoretically calculated from a system of simultaneous equations. The automatic equalizers solve these equations with the help of iterative algorithms, which produce results with sufficient accuracy for practical use after a finite number of iterations. After the initial setting is then the receiver assigned to the equalizer ready to receive data.

With an increasing number of common applications, high-speed data messaging is becoming transmitted in short bursts. Such uses occur in elective situations, including seat reservation of airlines, inventory control and banking systems. The data device start time limits the efficiencies Such systems seriously if they have the actual message transfer time reached or exceeded.

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The efficiency or efficients of a data transmission system can be defined with regard to the start time (neglecting the round trip delays) in the following way:

where T and T indicate the message time and the start time, respectively. A typical dial message with a length of 120 bits can be transmitted within a time T ₁ = 25 milliseconds at a speed of 4 800 bits per second. However, a representative data device (which is not set up for a quick start), but which is suitable for this transmission speed, requires the time T = 5,000 milliseconds-n for the start. High speed data devices themselves often require longer absolute start times because of their more complex carrier and timing controls, as well as their required equalizer control. In this example, 99.5% of the connection time is required for the message for the start (or start preparation), while only 0.5% is used for the actual transfer of information. The effective transmission speed is therefore only 24 bits per second. A cheaper, low-speed data device that does not require an equalizer could transmit the same message at a rate of 30 bits per second and still allow a full second of start-up time.

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For the economical use of high-speed data devices The total start time, including carrier phasing, timing recovery and equalization, must be on the order of magnitude held for about 10 to 20 milliseconds. It It has been shown that the parameters mentioned above depend on one another, but that the start-up time in practically operational systems is dominated by the warp convergence time.

The object of the invention is now the start time of data transmission devices to be kept low, in particular by making the convergence time of the equalizer low.

For a method of quickly obtaining the initial convergence the gain settings in a transversal equalizer of a synchronous data transmission system with a tapped delay line, which has an adjustable attenuator for each tap and with a summing circuit for selective combination the attenuated signals at the taps to form an equalized output signal is provided, the invention consists in that the following procedural steps are carried out in detail:

Receipt of a test word that traverses a distorting transmission medium as a periodic sequence

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has a number of bits per word that is identical to the number of taps,

Generation of a reference word as a periodic sequence, which is also one of the number of bits in the test word has an identical number of bits, but without initial synchronization of the test and reference word sequences,

bit-by-bit comparison of the test and reference word sequence at the output of the summing circuit to obtain a Error signal,

Correlation of the error signal to derive a group of tap coefficients and

cyclical shifting of the initial coefficients in order to achieve the greatest gain coefficient to a given one Create reference tap.

In an arrangement for carrying out the above-mentioned method, a transversal equalizer for rapid extraction having the initial convergence of the gain settings, the invention is characterized by

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a reference generator to generate the reference word,

a differential amplifier, for comparing the output signals of the summing circuit, for correlation of the error signal with the signals at the inputs of the attenuators for the creation of an orderly Group of initial coefficients

a maximum value detector to determine the largest tap coefficient and

a shifting circuit, for the cyclical shifting of the initial coefficients, for the alignment of the largest Tap coefficients on the reference tap.

Thus, according to the invention, the initial tap gain coefficients become generated very quickly for an automatic transversal equalizer in that, via the distorting transmission medium, whose interfering properties are to be compensated for, a periodic one with the data symbol speed Pseudo-random test patterns, the number of symbols of which is exactly equal to the number of taps of the equalizer, are transmitted

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thereby creating a local reference sequence in the receiver which is identical to the transmitted sequence with the exception of a likely time delay and where the transmitted Sequence, if it appears at the output of the equalizer, is compared with the local reference sequence. That from * such a comparison resulting error signal is increased with each equalizer tap signal correlated to a group of tap coefficients derive. These are cyclically shifted by the largest of these tapping coefficients at a specific reference tap to put on. To avoid a bias during the tap coefficient generation with respect to a specific tap, all tapping coefficients are preferably brought to the same receive setting, for example zero.

In accordance with one aspect of the invention, the Test sequences are generated continuously both on the transmit and on the receive side and at the equalizer output with the normal Speed, which is also used for message data transmission, compared.

According to a further aspect of the invention, a completely received test sequence is stored in the delay line, wherein the delay line is self-contained and the test

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follow continuously in the delay line at one rate which is preferably the same or much greater than the normal transmission speed. The local or reference sequence is generated at the same speed as the received test sequence.

The accelerated orbital speed is only due to inertia and limits the delay times inherent in the receivers and multipliers. A round trip interval the size of one Microsecond does not seem unrealistic, so that an equalization can be done extremely quickly.

Once an equalization has been achieved, synchronization between the test sequences takes place independently and independently from the normal or increased speed and also independently whether a group of tap coefficients that will achieve an open data eye pattern will also achieve it have another shift in place to align the largest tap coefficient with the equalizer tap, which is used as the reference tap was determined. Message data can now be sent from the equalizer at normal symbol speed in a adaptable operating mode can be processed, which reduces further residual distortion and slowly variable channel interference

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follows. This adaptable operating mode is realized by the extent to which the locally generated reference sequence is quantized form of the output signal of the equalizer when receiving data messages at normal speed will.

According to a further aspect, the received test sequence, the was subjected to the distorting effects of its passage through the transmission medium, stuck in the delay line and the tap connections with the tap reinforcement members are stored. As before, when the group will was determined from tap coefficients, the largest of these on the reference tap (usually the center tap) of the equalizer aligned.

Another feature of the invention relates to the rapid start-up speed of the automatic equalizer, which can be achieved with a minimal change in conventional structures.

Finally, another feature of the invention is that it can be implemented almost exclusively with digital circuits can be carried out.

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Further features, advantageous configurations and developments the method and the arrangement for carrying out the method according to the invention can be found in the subclaims.

The advantage of the present invention is that it is tremendous quick start division of the automatic transversal equalizer with synchronous data transmission systems.

Another advantage is that it is an ideal reference signal can be supplied in an automatic equalizer without the Prior synchronization is necessary.

It is also advantageous to improve the initial convergence more automatically Equalizer in heavily distorting channels thanks to the availability of an ideal reference variable.

Another advantage of the invention is a group of initial -Tap gain settings on an automatic To gain equalizers without first having to align them to the special taps.

Another advantage of the invention is the reduction in At the beginning, the start time of an automatic equalizer, which is arbitrarily dense

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the time it takes to receive and store a Test signal sequence lies, which has a number of elements which is equal to the number of taps of the equalizer.

Finally, one advantage of the invention is the ability to see an initial equalization in an automatic equalizer with a perform increased speed, which is much higher than the symbol transmission speed.

In the following an embodiment of the invention is based on the Figures described. Show it:

Fig. 1 "is a schematic block diagram of an auto

matic transversal equalizer, which according to the invention for a quick start was changed,

Fig. 2A - simplified block diagrams showing the con

2C

zept of the back-relationships between the circulating signal samples and the tap-attenuation arrangements of the automatic transversal equalizer according to the invention

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3 shows the block diagram of a further embodiment

example of the invention for a quick start system in an automatic transversal equalizer.

Automatic equalizers were developed by D. Hirsch and W. J. Wolf in that Essay "A Simple Adaptive Equalizer for Efficient Data Transmission" published in the "Transactions of the Institute of Electrical and Electronics Engineers on Communication Technology ", Volume Com-18, No. 1, February 1970, pages 5 to 12. Figure 3 of this article Figure 11 shows a root mean square equalizer, a non-recursive transversal filter structure that uses successive delays is equipped for an incoming signal which is equal to multiples of the symbol interval T. The signal samples taken from the delay line taps at intervals T. are selectively of adjustable tap coefficients in the boxes G attenuated and in an adder ': combined. The difference between the actual adder output and a quantized version of these signals forms an error signal part e at the output of a subtracter (indicated by a downward pointing Triangle). This error e is individually correlated in boxes X with the input signal and the tap signal samples, the mean value is formed in the boxes in order to generate control signals

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for the automatic setting of the gain coefficients of the Derive damper G in order to achieve optimal operation.

Assuming that only the received data eye pattern is open (observed on an oscilloscope, which is at the symbol speed is synchronized by superimposing successive signals) the error between the actual and quantized Output signals of the adder are adaptively reduced. More typically, however, the data eye is initially closed or it is lying at best on the sidelines. However, it is desirable to receive the entire message signal and not to lose such symbols, which would be used to achieve equalization. Therefore, an adjustment mode is always provided. During this setting mode becomes the reference signal necessary for the operation of the equalizer, usually with the help of the quantization of the Estimated output signal of the equalizer. However, it has been shown that the convergence behavior is seriously affected by this method can be when the channel distortion is significant and the initial error rate is high. To avoid this disadvantage and to provide an ideal reference signal, identical digital sequences can be used are synchronized in the transmitter and receiver, as explained, for example, in U.S. Patent 3,403,340. Unfortunately is the time required for synchronization between

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the two test signal trains, equal to the time it takes for the tap coefficient derivative is needed or will even surpassed.

Certain conditions have now been identified under which one Synchronization between two pseudo-random test sequences avoided so that that required for the tap coefficient generation Time has been reduced by several orders of magnitude. The conditions are (1) that the pseudo-random test sequences are exactly in their Symbol intervals coincide with the number of samples used in the equalizer, (2) which all tap coefficients on identical initial values are set and (3) that the largest tapping coefficient is determined and all coefficients are shifted cyclically to match the largest tap on the delay line, which is the reference tap was determined.

If, in addition, a completely received Pseudo-random test sequence the delay line is stored, the iterative comparison process with the reference test sequence for deriving the Tap coefficients are performed at any available rate, but preferably at a rate which is higher than the symbol transmission speed.

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Referring to Figure 1, the conventional part of the equalizer structure consists of a delay line having T delay elements 15 and with a tap 18 on the right side of each element. The equalizer also has an adjustable Attenuator 20 at each tap, an adder circuit 22 for combining the output signals of the attenuators on lines 21, a signal quantizer 26, a differential amplifier 28, a correlator 25 to which each tap 18 is assigned is, an integrator 24 with an atisgang for the control a certain attenuator and a data clock generator. The shown equalizer works with the baseband and is the Receiving side of a data transmission system assigned, which also consists of a transmission channel 13, a data source 11 and a data consumer 30. The shown equalizer works as described by Hirsch et al in the referenced paper, while message transmission is adaptive to slow changes to follow the channel parameters.

Before the message transmission, however, the initial setting for the attenuators 20 must be found in order to set the equalizer in to bring the adaptation area, i.e. the data eye must be open. Conversely, where conventional initial settings are one for the reference tap and 0 for all others, it is

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finding preferred, all initial settings on a common Bring value 0 or close to 0.

The data eye can be opened in the conventional way by the synchronized, pseudo-random number generators at the corresponding send and receive connections, as described by Hirsch has to be used. In typically, each of these generators provides test words with a greater length that corresponds to the number of Equalizer taps is unrelated. The generation of pattern lengths of 63 and 127 bits is common. Binary pseudo-random words can in shift registers with feedback connections between the last two stages and another stage, which is regarded as an input can be generated. The word lengths generated in this way are 2-1, where η is the number of shift register stages indicates. A three-stage data generator 36 in FIG. 1 represents a local pseudo-random generator or one which has a stored binary sequence can circulate. The test word generator 10 represents a similar generator to the one on the transmission side is arranged. The ideal reference signal from the synchronized local word generator is available improves the equalizer convergence, when the data eye is initially closed. In the known prior art it was assumed that this was the best correlation consists of longword synchronized sequences that start with the

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Symbol speed are clocked. However, it was found " that the word synchronization of the transmitter and receiver generated patterns can be dropped if the word length reaches the number is adapted to the taps of the equalizer, with all tap coefficients preset to identical values and finite Values of adjustment coefficients can be cyclically shifted in a correct order to match the adjustment coefficient align with the highest value on the reference adjustment. The shift distance is equal to the delay between the unsynchronized received and locally generated test sequences.

The principle of cyclic equalization is explained in FIGS. 2A to 2C. Each figure shows a delay line 15 with the symbol delays T and three taps. The test word will be first created at entrance 40. The output signals of the taps are selectively attenuated in adjustable networks 20 (shown as circles) and combined on output line 41. The feedback control, with the help of which the output signals 41 with an ideal Reference value are compared and the damping fun gs elements, which according to can be set using the well-known root mean square algorithm, are not shown.

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In Fig. 2A, the attenuators Cl, C2 and C3 were on a common Minimum value set until the delay line length word pattern in cells Tl, T2 and T3 of the delay line as a result Xl, X2, X3 was saved. At this time t, the first correlation is carried out with the ideal reference word, the consists of the undistorted elements Q1, Q2 and Q3 in this order, but which are not word-synchronized with the received one distorted consequence. Although there is bit synchronization, there is no frame or word synchronization. At time t ~ one thing becomes or other of the ideal elements are compared to the received summation on line 41. If the ideal element is Ql, the nominal corresponds to element Xl in the received sequence being sampled, then the equalization process produces the largest tap coefficient in the attenuator Cl in the cell Tl, where the element Xl of the received pattern is stored. In a similar way if it is the ideal element Q2 that nominally corresponds to the received element X2, the two patterns become random word synchronized and the largest tapping coefficient in attenuator C2 is correctly stored in cell T2 in which the received Element X2 is stored. Further, if it is the ideal element Q3 that nominally corresponds to the received element X3, then generated the correlation is the largest tap coefficient in the attenuator C3 in the cell T3 in which the received element X3 is

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stores is

Fig. 2B shows the received pattern by one unit of delay is shifted to the right, either along path 16, as shown, after the input is turned off or disconnected or via a new input, so that the element that was previously in the right place or the corresponding element moved to the left cell in a new word. Another sample of the ideal pattern is with the output signal on the Line 41 compared at time t + Tl. The ideal pattern is shifted by one delay unit so that the compared sample will be on the next line. The resulting processing of the shifted word patterns tends to converge the Values of the tapping coefficients towards an optimal combination, which differs from the combination specified at time t, however, their largest coefficient will remain in the same place. The received and ideal elements are further compared, e.g. at time t + 2T the order of the elements in the delay line 15 X2, Be X3, Xl. Finally, after a sufficient number of iterations the set of tap coefficients stored in attenuators 20 will be optimized. Like the relationships of the Whatever the ideal pattern received, it becomes one

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Group of properly ordered tapping coefficients are derived from the the above-mentioned operations.

Fig. 2C shows the state of the delay line 15 at the time of the Nth cycle after the tap coefficients have been optimized. The amplitudes of the tap coefficients are measured and the largest one aligned with the delay line reference tap. Thereafter, the message signal replaces the test signal on line 40 and the circulating device 16 is disconnected.

The embodiment of FIGS. 2A to 2C with three taps has been simplified too much for reasons of explanation. A three element test suite has too few random properties for practical use. Pseudo-random sequences of length 15 or 31 were made however, found to be sufficient when used in equalizers that have 15 or 31 taps, which is sufficient even for highly distorted channels.

Taking into account the above explanation of the principle of Invention, the operation of the circuit shown in FIG be easy to understand. In addition to the conventional elements numbered above FIG. 1, the equalizer includes switches 14, 27 and 33 with the respective positions A and B. The switches 14

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and 23 are optional and their use depends only on the desired mode of operation. The position A of all switches results in the known adaptive equalization, which develops a tap setting error signal from the difference between the actual output signal on line 23 of delay line 15 at summing circuit 22 and a normalized quantized output signal of quantizer 26. In the position A of the switch 14, the delay line 15 is loaded with a pattern length of the pseudo-random word which is sent out by the word generator 10 and received via the channel 13 at a transmission rate which is determined by the data clock 34. In position B, the switch 14 separates the input of the delay line 15 from the channel 13 and closes the feedback loop between the output of the delay element 15C and the input of the delay element 15A via the path 16. The switching line Π zn of the delay line 15 and the Local pseudo-random where generator 36 can be switched from the data clock generator 34 to the high-speed clock generator 35, which works a few hundred times or even a few thousand times faster than the data clock generator 34. The switch 27 transmits in the Switch position B, an input signal from the differential amplifier 28 from the output of the signal quantizer 26 to the output of the local word generator 36 via line 29. The equalizer

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now sets a root mean square error adjustment criterion with respect to an ideal rather than an estimated reference value.

In one aspect, the invention is practiced when the switch 27 are in position B and switches 14 and 33 are in position A (or, if the circuit is equivalent, without the Switches 14 and 33). In this aspect, successive test words are used without word synchronization at the normal data transfer rate compared. To the extent that the word periods are identical to the total delay periods of the delay line 15, equalization is achieved with very few word lengths. The tap coefficients are adjusted in a manner that causes the output of summing circuit 22 to close to the ideal reference sequence from the local word generator 36 sets. In this aspect, the random noise in the received word sequence tends to average itself to a minimum value.

In another aspect, the invention is practiced when the switches 14 and 27 in the federal position, the switch 33 is in the position A is located (or the circuit is with the data clock generator 34 constructed as the only clock generator). With this one

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Aspect, the delay line 15 is closed into itself after the first complete word is received and into the line has been entered, i.e. a ring circuit is formed via line 16, so that the output signal of the delay element 15C is fed back to the input of the delay element 15A. The only test word received is then repeated with the Reference test word compared to effect the equalization. With this aspect there is only a moderate influence of the noise interference the use of successive words received, but it is permissible because of the goal of a quick start, which is aimed in principle, in order to obtain an open data eye as quickly as possible and not necessarily in order to obtain optimal equalization.

In another aspect, the invention is practiced when all Switches 14, 27 and 33 are in position B. As soon as a complete word is stored in the delay line 1 5 was closed, the feedback loop via line 16. With the help of the switch 33 is now the switching line 17 with a fed accelerated scanning wave and causes the comparison of the received and reference test words with a high circulation speed, which is unrelated to normal data speed. The equalization is therefore in a slightly longer time than the Speieber time for a Test word length carried out.

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During equalization, either with normal or increased Speed, the attenuators 20 are free to assume new values. These values are achieved by eliminating the error differences between the accumulated tap output signals in the adder circuit 22 and the samples of the locally generated Test sequence from the generator 36 by the sensitivity control 31 via line 32 to correlators 25 to which the tap samples are also applied via line 19 be created. The adjustment coefficients are also effective in the integrators 24, either as voltages across a capacitor or stored as count contents in a counter.

As FIG. I shows with the dashed line 39, provision is also made for this made that these stored values circulate with the prevailing time cycle, controlled by the maximal value detector 45, the connected via line 46 to the output of reference integrator 24B is, in addition, the largest tap coefficient should be shifted. When the greatest signal appears at the output of the reference integrator, the cycle is stopped. The tap coefficients are then appropriately matched with the taps on the delay line 15 aligned so that leading and lagging echoes of the received Data signals can be compensated accordingly. The maximum coefficient can be localized in one cycle and the output

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direction can be made in a second. Because of this shift the tap coefficient, the subject of this invention is also referred to as cyclic equalization.

The sensitivity control 31 is provided for determining the size of the error difference which is to be correlated. The lower the setting of the controller 31, the more precise the equalization can be be optimized. However, the lower settings exceed the settling times for a given initial distortion. The switches 14 (if used) 27 and 33 (if used) are reset to position A and the message data of the Message data source 11 at the normal clock speed transferred, it should also be added that the switch 12 is in its B position. The adaptable equalization in one End mode, which is the quantized output signal of the quantization circuit 26 used as a reference value instead of the local test sequence of the generator 36 is now carried out to determine the residual distortion to further reduce and to track slowly occurring channel changes.

An alternative way of performing cyclic equalization is in Fig. 3 shown. The arrangement according to FIG. 3 is particularly advantageous where the type of delay line memory is largely one

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fast Umlaiif excludes. The delay line 15 shown in Fig. 3 can either be a capacitive delay line, in which the storage time is relatively short. The pseudo-random test sequence, once stored in the delay line of Figure 3, remains stationary. The taps 18A, 18B and 18C are assuming an equalizer with three taps, which is simple for the explanation, not permanently connected to their associated attenuators 20, but via synchronized switches that electronic switches are preferred because of the speeds in question. They are shown in the figure as mechanical switches shown with a common shaft. Each selector therefore has a rotatable input ε contact arm 43 and a number of output contacts, which are labeled A, B and C. All output contacts A are connected to bus 44A, contacts B are connected to bus 44B, and contacts C are connected to bus 44C. These collecting lines are in turn connected to the inputs of the adjustable attenuators 20A, 20B and 20C. The outputs of attenuators 22 on lines 21A, 21B and 21C become the Sximmie approximately and correlation circuits of the type mentioned in Fig. 1 continued. The rotatable contact arms 43A, 43B and 43C are synchronized via the link 42, the in turn with the normal time cycle or even higher speeds with the help of devices that are not shown in Fig. 3,

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to be turned around.

As soon as a complete test sequence in the delay elements 15 is stored in Fig. 3, equalization begins with all tap coefficients have been preset to equal values in order to avoid any setting of a particular tap as a reference. The correct flow of data to the summing amplifier is simulated by the cyclic rotation of the tap connections. As when the switches rotate, the test sequence appears in turn at the input of each attenuator 20. This mode of operation produces the the same effect as if the test sequence itself were to circulate in the delay line.

FIG. 3 also shows the maximum value detector 45, which monitors all tap coefficients of the attenuators 20. When the maximum is found, the sync link 42 can be stopped in a position that corresponds to the maximum coefficient at the reference tap 18B located. A similar dedector can also be used in the arrangement according to Fig. 1 used to get the largest tap coefficient to locate.

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Claims (4)

PATENT CLAIMS (^ 1 _> / Method for obtaining the initial convergence quickly the gain settings in a transversal equalizer synchronous data transmission system with a tapped delay line that has an adjustable attenuator for each tap has and with a summing circuit for the selective combination of the attenuated signals at the taps for formation an equalized output signal, characterized by the following steps: Receipt of a test word that has passed through a distorting transmission medium as a periodic sequence, which has an identical number of bits per word to the number of taps, Generation of a reference word as a periodic sequence, which is also one of the number of bits of the test word identical Has number of bits, but without initial synchronization of the test and reference word sequences, 2098A2 / 0748 22U398 bit-by-bit comparison of the test and reference word sequence at the output of the summing circuit to obtain an error signal, Correlation of the error signal to derive a group of tap coefficients cycling the tap coefficients to apply the greatest gain coefficient to a particular reference tap. 2. Arrangement for performing the method according to claim 1, with a transversal equalizer to quickly obtain the initial convergence the gain settings, indicated by a reference generator (36; Fig. 1) for generating the reference word, a differential amplifier (28) for comparing the output signals the summing circuit (19) for correcting the Error signal with the signals at the inputs of the attenuators for the creation of an ordered group of Initial coefficients, 209842/0748 a maximum value detector (45) for determining the largest Tap coefficients and a shift circuit (39) for cyclically shifting the tap coefficients to align the largest tap coefficient to the reference tap (18B). 3. Arrangement according to claim 2, characterized in that, furthermore a circulation circuit (14, 16; Fig. 1) for repeated circulation of a single received test word is provided from the output to the input of the delay line at a speed which is greater is than the synchronous transmission speed. 4. Arrangement according to claim 2 and / or 3, characterized in that the shifting circuit of a number of synchronized further Switch (43; Fig. 3) for the sequential correction of all taps (15; Fig. 1) of the delay line on the adjustable attenuators (20). 209842 / Q748